Dosimetric measurements will be performed with each new batch of iodine-125 sources, and further efforts made to refine the calculation of dose-rates at the interface between plastic and culture medium. The principal thrust of the biological investigations during the coming year will be the determination of the relative biological effectiveness (RBE) of the soft emission from encapsulated sources of iodine-125 compared with cesium gamma-rays, using Chinese hamster cells (CHO) cultured in plateau phase. These studies are planned for two reasons. First, plateau cells tend to accumulate in G1 and are considered to be a better model for the quiescent cells in a tumor than cells in exponential growth. Second, the use of plateau phase cells will eliminate cell division and make it possible to use lower dose-rates. It is planned to go down to 7 rad/hr of cesium gamma-rays and 4 rad/hr of iodine-125 radiation. The larger term goal will be to repeat the comparison of cesium and iodine-125 using short term cultures of human cells which in general exhibit curves characterized by a steeper slope and smaller shoulder than established cell lines of rodent origin. The response at low dose-rate will be studied and RBE determinations made.